It is usually assumed that at a liquid-solid interface the liquid has zero velocity. However, this condition is not always satisfied, and the liquid slips along the solid surface. This project tests the hypothesis that such slip plays a major role in reducing the cost of producing airflows in the insect respiratory system. Although theoretical considerations suggest that slip flow should exist in insect tracheae, its existence has never been studied. In mammalian and other blood-based circulatory systems, the shape of the red blood cells interacts with the cardiovascular geometry to reduce the friction in the distal portions of the vasculature, greatly reducing how hard the heart has to work to deliver oxygen to the body's cells. Insects use a different paradigm for oxygen delivery: they transport it directly to the tissues using a tracheal system, without the aid of blood. There is strong evidence to suggest that the insect system uses the slip to reduce energetic cost. The PIs will integrate computations and experiments to investigate the role of slip in insect respiration. The project will also provide interdisciplinary training for graduate and undergraduate students by immersing them in fluid mechanics and insect biology, and training them in experimental methods combined with rigorous theoretical and computational modeling. The results of this research will be disseminated in the classroom through courses that the PIs teach, in scientific publications, and at conferences, contributing to a generation of future researchers that promote cross-disciplinary research and technologies. Leveraging a recent NSF REU site award at Virginia Tech, three summer undergraduate researchers will work on this project. This REU partners with three HBCUs in Atlanta (Morehouse College, Spelman College, and Clark Atlanta University), and this connection will be used to recruit underrepresented students from those schools. Finally, both PIs are passionate about science communication and will actively engage the broader public via their Twitter and YouTube accounts, and previously established connections with journalists at print, web, and television venues like the New York Times, National Geographic, and the BBC.
Insect respiratory systems have evolved over millennia to handle gases at the microscale extremely efficiently, as evidenced by insects' unparalleled metabolic range. Without sophisticated active respiration strategies in the largest tracheal tubes, insects would likely be size-limited by diffusion. For the smaller tubes, the PIs propose that the mechanisms insects have evolved to enhance transport of gases and dissolved solutes deep into their bodies rely on passive respiratory actuation, in concert with asymmetric, three- dimensional respiratory geometries that are precisely tuned so that the air flow in them spans the continuum, slip, and transitional gas flow regimes at extremely low Reynolds numbers. The PIs have previously studied simplified mathematical and computation models of insect respiratory systems. Here, they propose studying the physics of gas transport in the smallest parts of actual insect respiratory systems directly for the first time. By uncovering the fundamental fluid mechanical strategies that insects use for transporting gases and solutes deep into the bulk of their centimeter-scale bodies, they will discover how nature solves the problem of actively distributing nutrients at the microscale, which will provide inspiration for new microfluidic devices and insight into the physics of respiratory flows at the smallest scales.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
